|
Looking at it again
Nanushuk/Torok discoveries push a rethink of North Slope oil potential Alan Bailey Petroleum News
New major oil discoveries in the Nanushuk and Torok formations on Alaska’s North Slope are causing some significant rethinking of the oil potential of the region, as companies move towards development of these exciting finds and people assess further exploration opportunities. Petroleum geologist Paul Decker from Alaska’s Division of Oil and Gas recently talked to Petroleum News about the nature and significance of the new finds. Decker sees the new Nanushuk/Torok oil play as opening the possibility of further significant oil discoveries to the west of the central North Slope. The play may also prove valuable as a geologic paradigm for oil prospects in the newly opened 1002 area of the Arctic National Wildlife Refuge, Decker thinks.
The three recent finds consist of the Pikka/Horseshoe trend in the Nanushuk in the Colville River delta region, the Willow discovery in the Nanushuk of the northeastern National Petroleum Reserve-Alaska and a major oil pool in the Torok under Smith Bay,
Oil Search in partnership with Repsol and Armstrong Energy is pursuing a development in Nanushuk sands in the Pikka unit - Armstrong also drilled the successful Horseshoe Nos. 1 and 1A wells in equivalent sands some 20 miles to the south of Pikka. During the last winter ConocoPhillips encountered oil in the Putu Nos. 2 and 2A wells, and in the Stony Hill No.1 well, all in that Pikka/Horseshoe trend. And ConocoPhillips is planning the development of its Willow discovery. Caelus Energy made its huge Smith Bay discovery in 2015 but thus far has not been able to further delineate the find or conduct any flow tests for the oil pool.
Brookian oil plays The Nanushuk and Torok form part of what geologists refer to as the Brookian sequence, the youngest and shallowest of the petroleum bearing rock sequences in Arctic Alaska. The sediments that formed the rocks were deposited down the margin of an ancient marine basin that filled from west to east, with the basin margin itself migrating in that same direction, as the basin filled. The more sandy sediments deposited in shallow water at the top of the margin later formed the Nanushuk formation, while the muddier and more layered sediments on the sloping side and on the floor of the basin formed the Torok formation.
Oil in the Nanushuk and Torok is found in what are called stratigraphic traps, hydrocarbon traps created through the disposition and sequence with which the sediments forming the rocks were laid down. The key to understanding the particular success of the Nanushuk play is the effect of cycles of sea level change and corresponding changes in sediment deposition, as the ancient basin margin migrated east, Decker explained. During periods of relatively low sea level, rivers dumped large quantities of sand out across to the basin shelf edge; it also appears that the sands were commonly reworked along the shelf edge to form elongated sand bodies, in some cases several miles long, such as is now observed in the Pikka/Horseshoe trend, Decker said.
Distinctive oil traps Then, as sea level rose, submerging the sands, the tops of the sand bodies were scoured off and impervious muddy sediments were deposited on top, to form what would later become hydrocarbon seals. In cross-section, the package of sediments took on a sigmoidal shape, pointed at top and bottom, with the sand at the top pinching out between mud above and below, and with the pointed end of the sand body directed towards the west, towards the source of the sediment pouring into the basin. Oil later accumulated in upper parts of the sand bodies.
Oil in the Torok formation is found trapped in interlayered shales and sands referred to as turbidites. And, given the more muddy nature of these sediments, deposited relatively far into the ancient basin, reservoir quality in the Torok tends not to be as good as in the Nanushuk. In addition to the Smith Bay discovery, oil has been found in the Torok in the Nuna and Moraine pools that straddle the Oooguruk and Kuparuk River units and in the Cassin discovery made in 2013 by ConocoPhillips to the north of the Willow discovery.
The sands with their potential hydrocarbon contents, particularly in the Nanushuk, tend to show up in seismic cross sections as seismic amplitude anomalies, thin bands along which the recorded seismic signals are particularly strong. And an examination of a northwest to southeast cross-section of the northern part of NPR-A reveals a multiplicity of these anomalies, with their distinctive shapes, curving down towards the east, marking the migrating eastward margin of the ancient basin.
Given that much of the North Slope, apart from the area of industrial development, is underexplored, the multiplicity of amplitude anomalies with their distinctive shapes raises an intriguing question over just how many oil pools remain undiscovered in the region, Decker commented.
Publicly available data And an examination of what is now known publicly about the new discoveries can also prove illuminating.
Maps of the structure and thickness of the sands in the Pikka/Horseshoe trend confirm that the sands would have been deposited just beyond the edge of the ancient marine shelf. Interestingly, a plot of oil pressure data from the various wells drilled in the trend demonstrates that fluid pressures have been communicating all through the trend across geologic time, Decker said. In other words, there appears to be significant reservoir continuity along the entire trend.
However, in detail, the rock quality does vary, so that there will likely be internal barriers to oil migration, with the hydraulic fracturing of horizontal wells being needed to encourage oil production, Decker added.
A plot of the water pressure gradient at Horseshoe juxtaposed on the oil pressure gradient for the trend suggests an oil/water contact at a depth of about 4,500 feet. That would imply the existence of an oil column more than 600 feet thick, Decker said.
The Willow discovery involves a large sand body that appears to have similar continuity to Pikka/Horseshoe across a considerable distance. Phillips Petroleum, later to become ConocoPhillips, originally made the discovery in 2002 from the drilling of the Hunter A well, a discovery that prompted the company to obtain some leases over the prospect in the 2002 NPR-A lease sale, Decker said. However, the company had other development interests at the time and did not return to the discovery until after Armstrong and Repsol had found and tested their Pikka discovery between 2013 and 2015. In 2016 ConocoPhillips confirmed the Willow trend through the drilling of the Tinmiaq No. 2 and Tinmiaq No. 6 wells, about 10 miles from the Hunter well.
Data from the two Tinmiaq wells, which have recently become public, appear to indicate lateral continuity across to the Hunter well in the sands that form the Willow reservoir. The data also indicate the presence of a younger and shallower sand body, referred to as the Falcon sand, that was not found in the Hunter well. It is not yet known whether the Falcon sand contains oil, Decker said.
Data for the Tinmiaq No. 2 well show that the reservoir interval contains fine grained sand and silt but that the reservoir quality is good, Decker said.
Light oil And the oil in the Nanushuk/Torok discoveries is relatively light, a factor that should help when it comes to oil production. The API density of the oil may relate to the oil source rock. The Pikka/Horseshoe oil is thought to have originated from a source in the Shublik formation and has an API of 42 to 43. The Willow oil has an API of around 30 - there is speculation that this oil may have come from a source rock known as the HRZ, or possibly from a Jurassic source, Decker said.
Unlike shallow oil to the east, found in sands above the legacy North Slope oil fields, the Nanushuk/Torok oil has not been degraded by microbes to form viscous or heavy oil. Decker speculates that, being at a deeper level within the geologic section, the Nanushuk and Torok may have been protected by impervious shales from water infiltration that might have promoted the degradation.
Because the maximum depth of burial of the main North Slope oil source rocks tends to increase towards the south, geologists tend to view the southern region towards the Brooks Range foothills as gas prone rather than oil prone. However, given the potential for lateral migration of fluids within the rocks, it is difficult to say how far south oil may be found in the Nanushuk/Torok play, Decker suggested. And, paradoxically, there is oil in the Nanushuk in the Umiat oil field, in the presumed gas province more than 40 miles south of Horseshoe.
ANWR implications With the west to east migration of the basin margin that formed the Nanushuk and Torok strata ending at around the location of the current Colville River, these rocks are not found to the east of this region However, a similar geologic process took place to the east, with much younger rocks forming as the ancient marine basin continued to fill. That raises some interesting questions regarding oil prospects in the 1002 area of ANWR.
The prospects in the younger Brookian sediments, deposited on the ancient shelf margin in the 1002 area, make a major contribution to the overall estimates of recoverable oil in the area. But the Nanushuk-style reservoir model, with reservoir sands pinching out towards the direction of origin of the sediments, has not really been evaluated, Decker said. Could this type of prospect add to the area’s oil potential?
Overall, with the new Nanushuk play proving a key to the opening of much more oil resource on the North Slope, a robust regional geologic framework is needed, to put everything into perspective and develop an effective exploration model, Decker said. The underexplored North Slope is still yielding exploration surprises, with the emergence of more assessed oil resources and the existence of many undrilled leads, he said.
|
